1. Field of the Invention
The present invention is related with an optical disc, an apparatus for reproducing the optical disc and a method of recording and reproducing the optical disc.
2. Description of the Related Art
As shown in FIG. 22, recording music information and image information onto an optical disc 92, is performed by converting it into a signal which has a parameter of pit length of an information pit 91 on an information recording surface 95. An apparatus for reproducing the music information and the image information from this optical disc 92, irradiates a laser light to the information pit 91 so that it may become a light beam spot LS having a predetermined diameter. And, it detects the diffracted and reflected light by a light detecting apparatus, which consists of a photodiode etc., outputs it as an electric signal, and extracts and outputs the recorded music information and the recorded image information from this electric signal, by giving signal conversion contrary to that at the time of recording.
There are a compact disc (CD), a laser vision disc (LVD), etc. as an optical disc used in this manner. In FIG. 22, a reference numeral 93 indicates a substrate which consists of polycarbonate resin etc., and a reference numeral 94 indicates a protection layer.
The information recording density of such an optical disc, is greatly influenced by an interval of the tracks, which are the center lines of the pit arrays of the information pits 91 shown in FIG. 23 i.e. the value of the track pitch P.sub.1 (=1.6 .mu.m: micrometers), and the value of the diameter of the light beam spot LS of the laser light irradiated onto the information pit 91 (=2.1 .mu.m). Therefore, in order to record more information on a disc, it may be considered to narrow the value of the track pitch from the presently available value.
However, although there arises no problem in examples shown in FIG. 22 and FIG. 23, for example, if a track pitch is set to a value P.sub.2 less than the conventional value P.sub.1 (for example, P.sub.2 =P.sub.1 /2) as shown in FIG. 24, information pits 91B and 91C etc. of neighboring tracks are included within the range of the beam spot LS of the laser light other than the information pit 91A which should be read. Therefore, under this condition, a crosstalk amount increases and it cannot be practically used.
On the other hand, it may be also considered to further reduce the diameter of the beam spot LS of the laser light. As shown in FIG. 25, a minimum beam diameter w at the time of condensing a laser light having a wavelength .lambda. at the position of a focal length f by an objective lens OL, becomes as a following expression (1). EQU w=1.22.times..lambda./NA (1)
Here, NA is a numerical aperture of the objective lens OL.
Apart from that, what is necessary is just to make the wavelength .lambda. less or enlarge the numerical aperture NA, in order to make the laser beam diameter w less. As for the wavelength .lambda. of the laser light, the wavelength of a semiconductor laser presently used .for the optical disc, is about .lambda.=0.780 .mu.m (.mu.m is 10.sup.-6 m). Moreover, as for the numerical aperture NA, it is about 0.45 in case of the CD. Thus, the minimum value w.sub.min of the laser beam diameter, becomes about a value as expressed by a following expression. EQU W.sub.min =1.22.times.0.780/0.45=2.1 (.mu.m)
Therefore,. the minimum track pitch, which does not generate a crosstalk when the laser beam spot is irradiated on a certain pit array, becomes about 1.6 .mu.m, which value is employed by many present optical discs.
Therefore, under the conditions of the laser light wavelength and the lens numerical aperture which are presently used, in order to further improve the information recording density, it becomes necessary to raise the amount of information of each information pit itself.
As one method for this, it may be considered to divide the information pit into two or more information pit pieces, and change the arrangement and combination of the information pit pieces.
In this case, in order to reproduce the record information, it becomes necessary to distinguish the information pit not only as for the presence and the absence but also as for the disposed position of the information pit pieces. For this reason, the influence of the positional offset and the adjacent information pits should be restrained as much as possible. Moreover, with respect to the light detecting apparatus to reproduce the information, a device is requested which can restrain the influence of the positional offset and the adjacent information pit, and can output more accurate detection signal.
In this manner, it is difficult to improve the recording density and at the same time perform a reliable reproducing operation of the optical disc.